This invention relates to an improvement in a process for preparing tetrahalophthalic anhydride whereby there is a substantial recovery of reactants and by-products such as halogen, SO.sub.3, and SO.sub.2, from a vent stream exiting the reaction vessel.
Methods for the formation of tetrahalophthalic anhydride, particularly tetrabromophthalic anhydride and tetrachlorphthalic anhydride are well known. A widely used method for halogenating phthalic anhydride is disclosed by Jenkner et al. U.S. Pat. No. 3,382,254 wherein bromine is reacted with phthalic anhydride in a molar ratio of 2.01 to 2.1 at temperatures of 80.degree. to 110.degree. C. in the presence of a small quantity of halogenation catalyst and 4.4 to 4.8 moles of sulfur trioxide in the form of 50 to 80% oleum per mole of phthalic anhydride. After completion of the addition of bromine, the excess sulfur trioxide and bromine are distilled off from the reaction mixture by heating the reaction mixture to 130.degree. to 145.degree. C.
While a considerable amount of the sulfur trioxide and bromine distilled from the reaction vessel can be condensed and recycled to the reaction vessel, there remains a non-condensible stream containing a substantial amount of bromine and SO.sub.3 which cannot be economically recovered by conventional techniques. Since SO.sub.3 has such a high melting point, solid SO.sub.3 forms readily upon cooling the vent stream, thus making separation and handling of the vent stream more difficult. Until now, neutralization of this non-condensable reaction vent stream has been the only viable means for removing substantially all of the SO.sub.3 and bromine before discharging the vent stream to the atmosphere.
Thus an object of this invention is to provide a facile economic means for removing essentially all of the residual halogen and SO.sub.3 in the reactor vent stream before exhausting the vent stream to the atmosphere. Another object of this invention is to reduce the amount of purchased oleum by using the recovered SO.sub.3 from the reaction vent stream to increase the concentration of oleum in a dilute oleum solution. Other objects of this invention will be evident from the following description and appended claims.